Petroleum-derived polyester (hereinafter referred to as “polyester”) such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN) is known as high-performance/high-functional plastic which is excellent in dynamic characteristic, heat resistance, transparency, and the like, as compared with polyolefin which is commodity plastic. In particular, PET is inexpensive and is therefore used in large amounts (annual production in Japan: approximately 700,000 tons) in various kinds of bottles, containers, industrial products, industrial parts, and the like, and is also famous as a recyclable substance. Moreover, PBT, PEN, and the like are classified into engineering plastic which is a high-performance resin. Here, the engineering plastic is defined as a resin whose heatproof temperature is 100° C. or higher, tensile strength is 50 MPa or more, and tensile modulus of elasticity is 2.5 GPa or more. Further, engineering plastic whose heatproof temperature is 150° C. or higher is called “super engineering plastic” and there is a growing demand for such super engineering plastic in the field in which higher heat resistance of electronic apparatuses and the like is requested.
In order to improve mechanical characteristics (such as tensile strength and tensile modulus of elasticity) and heat resistance of polyester such as PET, a polyester sheet and a polyester film (stretched sheet and stretched film) are used which are obtained by subjecting polyester to stretching treatment. However, in conventional polyester stretched sheet products and polyester stretched film products, the high performance intrinsic to polyester is not sufficiently achieved. This is because the conventional polyester stretched sheet and polyester stretched film have a laminated lamellar structure in which folded chain crystals (FCC) and amorphous materials are laminated, and contain the amorphous material which is low in performance at a ratio of 50% or higher. For example, although a conventional uniaxially-stretched PET sheet has a tensile strength (σ) of 230 MPa and a tensile modulus of elasticity (Et) of 2.3 GPa at a room temperature, a heatproof temperature (Th) of the conventional uniaxially-stretched PET sheet is approximately 120° C., which is notably lower than a melting point (Tm) of 250° C. to 265° C. and an equilibrium melting point (Tm0) of 310° C. of PET. Therefore, the conventional uniaxially-stretched PET sheet has been difficult to use in the field in which high heat resistance is requested. This fact has prevented PET from being fully employed in industrial products.
Here, Patent Literatures 1 and 2 and Non-Patent Literatures 1 through 3 disclose techniques in relation to a uniaxially-stretched sheet of polyester such as PET. However, the uniaxially-stretched polyester sheets disclosed in those literatures all have the above described laminated lamellar structure. FIGS. 4 and 6 of Non-Patent Literature 1 illustrate a typical 4-point image which indicates that the uniaxially-stretched PET has a laminated lamellar structure based on a small-angle X-ray scattering pattern (SAXS pattern) of the uniaxially-stretched PET. Moreover, FIG. 8 of Non-Patent Literature 1 schematically illustrates a uniaxially-stretched PET sheet which has a laminated lamellar structure in which amorphous materials and crystals are laminated. Moreover, FIG. 5 of Non-Patent Literature 3 also illustrates a small-angle X-ray scattering pattern (SAXS pattern) of uniaxially-stretched PET by a typical 4-point image of a laminated lamellar structure.